1. Field of the Invention
The present invention relates to a method of doping silicon bodies through the diffusion of boron into the bodies. A borosilicate glass layer is applied to the partially masked surface of the silicon bodies. Beneath the borosilicate glass layer, an SiB.sub.6 layer is formed on the unmasked areas. The silicon bodies are exposed to a gas mixture containing a predetermined boron quantity and boron and oxygen in a predetermined quantitative ratio to form the SiB.sub.6 layer and the borosilicate glass layer. During a heating process at least part of the boron is driven into the silicon. Prior to the heating process the borosilicate glass is completely stripped off without affecting the SiB.sub.6 layer.
2. Prior Art
Capsule diffusion methods are known for diffusing boron into silicon wherein silicon bodies are exposed to a gas which flows through a reaction tube and which contains a boron source. With capsule diffusion methods, highly homogeneous and reproducible results can be obtained. Capsule diffusion methods, however, have disadvantages related to the amount and complexity of material and apparatus required. Compared with capsule diffusions, boron diffusions by means of a boron source-containing gas stream flowing past the silicon bodies, i.e. so-called open diffusions, require a much smaller amount of apparatus and material. In these open diffusions, a boron glass is produced on the surface of the boron-doped silicon body since the highly heated silicon bodies are exposed to a gas stream containing an oxidizing boron.
An open diffusion method is described in German Offenlegungsschrift No. 23 16 520. In the method described, a borosilicate glass layer is applied on silicon wafers in an inert atmosphere during a first heat treatment. Simultaneously, a phase (SiB.sub.6) containing silicon and boron is provided between the borosilicate layer and the silicon surface and a thin layer highly doped with boron is generated in the silicon surface. Toward the end of the first heat treatment, the silicon wafers are exposed to an oxidizing atmosphere for a predetermined period during which the SiB.sub.6 phase is oxidized. The borosilicate glass layer and the layer of oxidized SiB.sub.6 phase are stripped in an etchant. Subsequently, the boron from the thin highly doped layer is driven into the silicon in a second heat treatment. Although this method provides homogeneous and reproducible results in the fabrication of low-doping diffusion areas within very close tolerances over the silicon body and throughout the entire batch, the results obtained are not fully satisfactory in view of the increasing trend toward closely packed highly integrated circuits.
Another disadvantage of the method described in the aforementioned Offenlegungsschrift is that during oxygen treatment after boron oxide coating crystal dislocations are formed in the doped silicon areas and in the overlapping areas of zones doped with boron and arsenic. Frequently, these dislocations are the cause of short-circuits in bipolar transistors.
In order to remedy these disadvantages, German patent application No. P 28 38 928.8 suggests a diffusion method for boron doping of silicon, which saves time and costs and provides very homogeneous and highly reproducible results. Before the method described in this patent application had been invented it was thought that high diffusion results required that the SiB.sub.6 phase had to be removed prior to the drive-in of the boron in the second heating process (K. M. Busen et al. in J. Electrochem. Soc., Vol. 115, March 1968, pages 291).
Surprisingly, it was found that it was possible to provide very homogeneous and highly reproducible results within very close tolerances, by completely stripping only the borosilicate glass without substantially affecting the SiB.sub.6 layer prior to the second heating step.
A disadvantage of this method is that the borosilicate glass cannot be completely stripped with diluted hydrofluoric acid. The borosilicate glass can, however, be stripped by immersing the silicon wafers first for ten seconds in diluted hydrofluoric acid, then for 10 minutes in concentrated nitric acid heated to 95.degree. C., and finally for another 30 seconds in diluted hydrofluric acid. The SiB.sub.6 phase is not appreciably attacked in this stripping process, but the etching solution used for stripping the borosilicate glass has a number of disadvantages: The use of concentrated nitric acid heated to 95.degree. C. causes environmental and safety problems. The hot nitric acid evaporates and is sucked off during the etching process so that nitrous gases reach the atmosphere. Furthermore, the apparatus used is exposed to corrosion. Therefore, it would be desirable to replace the hitherto used etching cycle for fully stripping only the borosilicate glass by a less dangerous process.